Harmonic imbalance is a combination of tortional reaction and engine imbalance but I doubt is is your problem here. You say you are a thou here and a thou there and although it may not seem much, it will have the effect you describe. Tkg was right in his quote about vibration and how it tends to smoooth out at a particular rpm which is why they re-balance stock engines when they are to be used at higher rpm. All engines vibrate at particular rev ranges and oddly enough, odd number cylinder engines are smoother than even numbered cylinder.Normally external balance of a crankshaft is avoided in favour of internal balancing as the former tends to add another dimension of imbalance.
In short, your hubs need to be true turned. I have heard of a Gnats whisker having been used many times before as a measurement , even used it myself to describe a small amount, but so far cannot track down any reference anywhere as to the actual size. Maybe it is rellavent to the job at hand and therefore varies.